This invention relates to photoflash lamps and, more particularly, to an improved method for applying a polymeric coating on the glass envelope of a flashlamp.
A typical photoflash lamp comprises an hermetically sealed glass envelope, a quantity of combustible material located in the envelope, such as shredded zirconium or hafnium foil, and a combustion supporting gas, such as oxygen, at a pressure well above one atmosphere. The lamp also includes an electrically or percussively activated primer for igniting the combustible material to flash the lamp. During lamp flashing, the glass envelope is subject to severe thermal shock due to hot globules of metal oxide impinging on the walls of the lamp. As a result, cracks and crazes occur in the glass and, at higher internal pressures, containment becomes impossible. In order to reinforce the glass envelope and improve its containment capability, it has been common practice to apply a protective lacquer coating on the lamp envelope by means of a dip process. To build up the desired coating thickness, the glass envelope is generally dipped a number of times into a lacquer solution containing a solvent and a selected resin, typically cellulose acetate. After each dip, the lamp is dried to evaporate the solvent and leave the desired coating of cellulose acetate, or whatever other plastic resin is employed.
In the typical dipping process for applying protective coatings, a large number of flashlamps are loaded on a rack and then successively dipped in a solvent solution and oven dried three or four times to build up the desired coating thickness. Such a process is time consuming, uses a relatively large area of production floor space, and involves considerable hand labor, all of which add significantly to manufacturing cost. Further, as the lacquer solution includes a highly flammable solvent, such as acetone, an inadvertent flashing of one of the lamps in either the dip bath or drying oven can ignite the solvent fumes. To control this hazard safely, costly automatic extinguishing equipment must be employed. In the event of a solvent ignition, the resulting downtime and consumption of fire extinguishing chemical also adds to the manufacturing cost.
Application of the protective coating by means of a dipping process can also preclude the use of more desirable reinforcing materials. For example, a much stronger containment vessel could be provided by the use of a polycarbonate coating, due to its higher impact strength and higher softening temperature, as compared to cellulose acetate. By using the conventional dipping and drying process to apply polycarbonate, however, a relatively cloudy coating results. In order to obtain a clear, transparent coating, an extremely low humidity must be maintained in the drying ovens, which in turn requires the drying of 5000 to 10,000 cubic feet of air per minute. The incorporation of such a drying operation would be prohibitively expensive.